Here, it runs without errors, but after all of the data is transferred, it just keeps copying logs and never creates the _checkpoint file, so I have to kill the screen session and xtrabackup_51 manually.

Googling this points me to believe the databases are corrupted after trying many fixes including increasing the timeout (it was already 28800). Could this have been caused by xtrabackup failing to finish and create the checkpoint file or not copying the .frm files itself? I've attempted this multiple times on multiple ubuntu servers I've built from scratch. Any suggestions or a bug fix for xtrabackup?